The positioning of a raster scan line, like that generated by the optics of a polygon laser printer onto a photosensitive image recording medium involves assuring accurate alignment in the x (cross-scan) and z (beam path) translational directions, as well as in the .theta.x and .theta.z rotational directions respectively about each of the x and z directions. Proper positioning also requires attainment of the right angle of incidence of the beam onto the target medium.
Excessive focal shift errors of the scan line with respect to the recording medium can lead to a printing artifact called banding. Banding is an undesirable periodic density modulation in the image that is most noticeable in uniform density regions. A very high degree of accuracy of scan line placement is required especially for laser printers used for x-ray films, because banding artifacts can lead to false diagnoses. The eye is very sensitive to low frequency (1-8 cycles/degree) contrast modulation, having a threshold as low as 0.2% under the right illumination conditions. The design goal typically will use half this tolerance, so it will have 0.1% allowable line shift for the extreme polygon facet error. For a printer with a line pitch of 0.08 mm, the allowable error is thus only 80.times.10-9 mm. This requirement means that only a very small focus error can be present for the page cross section of the beam.
Scan line generating optics systems are susceptible to two types of focusing errors: those which cause deviations of the scan curve from a straight line; and those which displace the scan line from its intended generatrix line on the target drum. The f-.theta. condition correcting and other optical elements can be manufactured with sufficiently tight tolerances and alignment procedures to ensure the generation of an acceptably straight (i.e. non-arcuate) scan line in space.
U.S. Pat. No. 5,214,441 issued May 25, 1993 to Blanding et al., discloses a method and apparatus for ensuring the proper placement of a scan line generated by the optics of a polygon laser printer onto a recording medium supported on a rotating drum. An inexpensive mechanism is provided for setting the x, z, .theta.x, .theta.z and angle of incidence positioning of the scan line. Beam source, shaping and scanning optics are mounted on a rigid module and adjustable means is provided to mechanically rigidly connect the optics module to the drum. The connection is provided by two inverted V-notched blocks that are selectively positionable in an alignment fixture relative to the module frame and which are brought into nesting relationship onto cylindrical surfaces of bearings mounted coaxially with the drum shaft. The V-notched blocks are rigid plates and rely on gravity, and possibly spring clips connected between the module and the frame of the printer to augment the nesting force between the V-notched blocks and the cylindrical bearing surfaces. It has been found that acceleration forces generated when the printer is moved, for example during shipping, and/or the clamping forces generated by the spring clips can result in excessive stress and deformation of the V-notched block material at the line interfaces between the cylindrical bearing and the V-notched blocks. Any such deformation results in a corresponding alignment shift of the scan optics with respect to the imaging drum.
There is a need therefore for an improved method and apparatus for accurately positioning scan line generating optics relative to an intended scan line receiving target medium in a laser scanner.